1. Field of the Invention
The present invention relates to a method and an apparatus or equipment for manufacturing pigment dispersion by continuously flushing pigment press-cake by using a twin screw extruder, and more particularly relates a method and equipment for continuously flushing and dehydrating in the extruder in order to obtain effectively high quality, namely low water content, pigment dispersion having fine pigment particles.
2. Description of the Prior Art
Conventionally, methods for manufacturing printing inks, paints and various conditioned colorants for plastics were roughly divided into processes of using powder pigments and processes of using pigment press-cakes. In the former case, since the pigment elementary particles coagulate firmly in the drying process, it is difficult to disperse them into elementary particles even by subsequent strong dispersion or kneading, which often results in inferior color value and transparency. In the latter case, an organic medium such as a water-immissible binder is added to pigment particles in the pigment press-cake which do not coagulate as much, and the mixture is violently kneaded to flush the pigment particles from the water phase to the organic medium phase, which is used as pigment dispersion, so that the pigment in a state close to elementary particles may be directly used. As a result, the color value and transparency are excellent, and this method is used widely.
However, conventional flushing is carried out by the so-called batch process, which involves the following problems. For example, when producing printing inks, pigment presscakes and ink binders are charged into a flusher possessing a trough of large capacity, and agitator blades are powerfully rotated with a large power to knead and flush the pigment. Then water is removed by decantation, and the residue is heated and dehydrated under vacuum to obtain pigment dispersions.
In this method, very large equipment is required as compared with the final output of pigment dispersions, much time and large power are necessary for kneading and flushing, the flushing product is in dough form which is low in efficiency of vacuum dehydration and takes a long time for dehydration, and the actual water content is around 3%, even if dehydrated for a long period of time, and is difficult to reduce below 1%. Since the batch system is employed, it is difficult to obtain flushing products of a uniform quality. Additionally, the operating procedure is complicated, and it is practically difficult to obtain high pigment loaded dispersions.
As a means of solving these problems of batch process flushing, there are proposals relating to continuous flushing. For example, Japanese Patent Application Public Disclosure No. 50-51532 discloses using a line mixer. According to this method, although the problems peculiar to the conventional batch process can be solved, since the line mixer is a static mixing device and continuous flushing is possible, dehydration is achieved by batch processing (again it is difficult to lower the water content below 1%). Hence this method is not a completely continuous process. Besides, because the line mixer is a device to continuously agitate and mix fluids within its transport line, it is essential that the object possess fluidity, and it becomes necessary to dilute and adjust the pigment press-cakes or to add special surfactants, which involves problems in efficiency and product quality.
In the conventional batch process flushing, since the materials are processed in one container, feeding problems do not usually occur, but it is important in the continuous operation to feed each material uniformly while maintaining a constant feeding flow rate.
The conventionally used pigment press-cakes are wet cakes with pigment content of 15 to 35% presenting a plaster-like form, and are prone to plastic deformation, lacking in uniformity of pigment content. This form is very disadvantagenous for constant feeding. Therefore, emphasizing only the continuous constant feeding property, it is sufficient to dilute the cake in water to feed in surry form as shown in the aforesaid Japanese Patent No. 50-51532. But in this method, the treating volume increases and the flushing efficiency decreases, which may lead to lower productivity.